in redshift space, where \(P_\mathrm {L}(k)\)  is the linear power
spectrum, \(\mu = \hat{{z}}\cdot \hat{{k}}\)  is a directional
cosine of the wavevector \({k}\)  with respect to the line of sight
\(\hat{{z}}\) . This resummation factor strongly suppresses the power
on small scales [1]}, \(k \gtrsim k_\mathrm {NL}\) 
with
\(k_\mathrm {NL} \equiv [(6\pi ^2)^{-1} \int dp P_\mathrm {L}(p)]^{-1/2}\) ,
where the perturbation theory breaks down and does not have a
predictive power. Some people tend to extrapolate the results of
perturbation theory into a strongly nonlinear regime where
dimensionless power spectra or the correlation functions are no longer
small, and the proper perturbation theory is not expected to be
applicable. When the proper orders of the perturbation theory are
kept, one can expand the exponent in Eqs. (REF ) or
(REF ), and truncate at a desired order. Phenomenologically,
however, the fingers-of-God effects, redshift-space distortions by
random motions on small scales, are favorably represented by
exponential form in Eq. (REF )
[2]}, [1]}.
